The invention relates to an alkaline earth metal carbonate in particle form, and to its production and its use.
Calcium carbonate, barium carbonate and strontium carbonate are used for many technical purposes, for example as starting material for the synthesis of other calcium, strontium and barium compounds. The carbonates are suitable also for use in solid reactions, such as the preparation of oxide-contain ceramic materials for electrical and magnetic applications, e.g., for the manufacture of thermistors and condensers; they are used for the production of glass and for the manufacture of high-temperature superconducting material. Calcium carbonate is also used in paper manufacture.
Japanese patent application 7/025611 discloses a micronized barium carbonate which has a surface area of 5 to 25 m2/g. For its production an aqueous solution of barium hydroxide, to which carbon dioxide is added, is passed through three pumps in tandem and adds to the reaction product leaving the third pump a carboxylic acid or carboxylic acid derivative such as carboxymethylcellulose, which is intended to prevent crystal growth.
The present invention is addressed to the problem of devising an effective method for the preparation of micronized calcium carbonates, barium carbonates and strontium carbonates in the form. of particles with a narrow grain distribution. This problem is solved by the method of the invention.
The method of die invention for the preparation of particulate alkaline earth carbonate MCO3, wherein M represents Ca, Sr or Ba, provides for adding carbon dioxide to an M(OH)2 solution with a concentration of 0.1 to 0.75 mol/l of M(OH), with the formation of a reaction mixture, using about 2 to 30 l of carbon dioxide gas per liter of solution, passing the reaction mixture through a continuously running mixing reactor in which shearing, thrusting and friction forces of interacting tools with a high relative velocity act upon the reaction mixture according to the rotor-stator principle, and separating and drying the alkaline earth carbonate formed from the reaction mixture after it passes through the reactor. The shearing, thrusting and friction forces in the reactor bring it about that the carbon dioxide enters in extremely finely divided form into the alkaline earth metal hydroxide solution. The liters of the carbon dioxide gas referred to are liters under standard conditions.
In the practice of the method of the invention micronized alkaline earth metal carbonate is obtained in which at least 90% of the particles have a diameter ranging from 0.1 to 1.0 xcexcm, preferably ranging from 0.2 to 1.0 xcexcm in the case of barium carbonate or strontium carbonate. In the case of calcium carbonate, at least 90% of the particles have a diameter ranging from 0.2 to 1.0 xcexcm. In barium carbonate the BET surface area ranges from 3 to 30 m2/g, preferably 3 to 20 m2/g, especially 8 to 15 m2/g. In calcium carbonate and strontium carbonate the surface area ranges from 3 to 50 m2/g.
The method of the invention is very efective. Only one reactor is needed in order to achieve a virtually quantitative reaction. In the method of the state of the art, three pumps are used in tandem.
Very suitable are apparatus in which the rotor revolves at a high speed. The rotor speed amounts preferably to 2,000 to 8,000 revolutions per second. The time of stay of the reaction mixture in the mixing and homogenizing apparatus amounts preferably up to 5 sec.
In this manner, BaCO3 for example, can be produced, wherein 90% and more of the particles have a diameter ranging from 0.2 to 0.7 xcexcm.
Measurement of the BET surface area was performed with an apparatus able to measure micropores, of the firm, Micro Meritics. Adsorption gas was nitrogen, and the measurement was made at the temperature of liquid nitrogen in the range of 0.01-0.1 Prel using the BET equation.
In one embodiment of the invention the method is practiced without the addition of a crystal growth inhibitor This embodiment will be further explained in connection with the preferred production of BaCO3.
Before the drying operation, which is best performed in the range of 100 to 120xc2x0 C., the barium carbonate separated immediately after passing through the reactor, can be washed one or more times with water.
The precipitation of barium carbonate in the barium hydroxide solution treated with carbon dioxide is best performed at a temperature in the range from 40 to 130xc2x0 C.
The addition of agents intended to prevent crystal growth in the reaction mixture leaving the homogenizer is not performed; the carbonate is separated immediately after leaving the reactor.
Additional subject matter of the invention is the particulate barium carbonate, which is obtainable by the method of the invention and has a BET surface in the range of 3 to 30 m2/g, preferably 3 to 20 m2/g, especially 8 to 15 m2/g, in which at least 90% of the particle has a diameter ranging from 0.2 to 1.0 xcexcm, and which is free of agents preventing crystal growth.
Additional subject matter of the invention is particulate calcium carbonate with a BET surface ranging from 3 to 50 m2/g, wherein at least 90% of the particles has a diameter ranging from 0.1 to 1.0 xcexcm, preferably 0.2 to 1.0 xcexcm, obtained by the method of the invention, which is free of agents preventing crystal growth.
Subject matter of the invention is also particulate strontium carbonate with a BET surface ranging from 3 to 50 m2/g, wherein at least 90% of the particles has a diameter ranging from 0.1 to 1.0 xcexcm, obtained by the method of the invention, which is free of agents preventing crystal growth.
Another embodiment of the invention provides that an agent preventing crystal growth is added to the reaction mixture before, during and/or after the reaction in the reactor. In this embodiment also, calcium, strontium and barium carbonates can be made with the above-mentioned characteristic of particle diameter and specific BET surface area. It is preferentially suited for the preparation of barium carbonate and strontium carbonate. This preferred embodiment will he further explained; special crystal growth inhibiting agents are used which are given preference. Also usable would be the acids themselves or the alkali metal salts, for example.
The particles occur in rod shapes. The statement of the diameter of the particles relates to the length of the rods.
The preferred method of the invention for the preparation of particulate barium carbonate with a BET surface area ranging from 3 to 30 m2/g, preferably 3 to 20 m2/g and strontium carbonate with a BET surface ranging from 3 to 50 m2/g, wherein at least 95%, preferably at least 90%, and especially 100% of the particles have a diameter xe2x89xa61.0 xcexcm, provides that, with the use of an agent preventing crystal growth selected from the group of the ammonium salts and alkyl ammonium salts of carboxylic acid having a total of 3 to 12 carbon atoms as well as at least 2 COOH groups, or at least 2 OH groups, a barium hydroxide solution or strontium hydroxide solution with a concentration of 0.1 to 0.75 mol/l of Ba(OH)2 or Sr(OH)2, respectively, is treated with carbon dioxide with the formation of a reaction mixture, using about 2 to 30 l of carbon dioxide gas per liter of solution, the reaction mixture is passed through a mixing reactor in which shear and friction forces act upon the reaction mixture. The shear and friction force in the reactor cause the carbon dioxide to enter in extremely finely divided form into the barium or strontium hydroxide solution. The liter specification (mainly 10 to 20 l in the processes) of the carbon dioxide gas refers to standard conditions. If 100% of all particles have a diameter xe2x89xa61 xcexcm, this means that no more than very minor undesired amounts, e.g., less than 0.1% of the particles, have a diameter greater than 1 xcexcm.
The agent preventing crystal growth is used at least in the amount that limits the crystal growth of the barium carbonate and strontium carbonate such it at least 99% of the particles have a diameter of xe2x89xa61 xcexcm, e.g., in the range from 0.2 to 1.0 xcexcm. The amount of the agent can best be at least 0.01 wt.-%, preferably at least 0.1 wt.-% of the BaCO3 taken as 100 wt.-%, or SrCO3, as the case may be (dry substance). It can, for example, be in the range from 0.01 to 5 wt.-% of the barium carbonate or strontium carbonate taken as 100 wt.-%. Preferably, the carboxylic acid salt affecting crystallization is used in an amount of 0.2 to 0.7 wt.-% of the barium carbonate or strontium carbonate taken as 100 wt.-%. Since the carboxylic acid salt can be used before, during or after the carbonization, it is possible to reckon the amount of carbonate to which the carboxylic acid salt amount relates by assuming a 100% reaction of the barium hydroxide or strontium hydroxide.
With special preference, ammonium salts of the carboxylic acid are used. However, alkyl ammonium salts of the carboxylic acid can also be used; these can be primary, secondary, tertiary and quaternary alkyl ammonium salts. For example, primary, secondary and tertiary ammonium cations are suitable, the nitrogen atom being substituted by alkyl groups with a total of up to 18 carbon atoms. Methyl, dimethyl, trimethyl, ethyl, diethyl, triethyl, n-propyl, di-n-propyl and tri-n-propyl ammonium salts, as well as ammonium cations with different alkyl groups as substituents, are usable, for example.
Salts of carboxylic acids with a total of 3 to 12 carbon atoms, preferably 3 to 6 carbon atoms, are used, as stated; the carbon atoms of the carboxyl group are included. Especially well suited are ammonium and alkylammonium salts of malic acid, adipic acid, citric acid, gluconic acid, glucaric acid, glucuronic acid, tartaric acid and maleic acid. Preferred are the ammonium salts, especially citric and maleic acid. The invention shall be further explained with the aid of this preferred embodiment.
Ammonium salts of citric acid and maleic acid are used preferentially, as stated, especially those ammonium salts of citric acid and maleic acid which, when in aqueous solution, result in a pH ranging from 6 to 8. They can be prepared by adding ammonia or spirit of saimiac, until a pH ranging from 6 to 8 is reached.
The citrate salt or malcate salt is advantageously used in the form of an aqueous solution. The citrate or maleate concentration amounts advantageously to at least 10 wt.-%. It can even have a saturated concentration.
It is preferred that the carboxylic acid salt be admixed immediately after the reaction mixture has passed through the apparatus used for the carbonization. Thus the formation of undesirably large crystals of barium carbonate or strontium carbonate is especially effectively prevented. The immediate separation of the carbonate is not necessary.
Before the drying, which is best performed in the range of 100 to l 50xc2x0 C., the carbonate separated after passing through the homogenizer can be washed one or more times with water. Thus, ammonium salts (e.g., carbonates of ammonium or alkyl ammonium) are washed out.
The precipitation of barium carbonate in the barium hydroxide solution treated with carbon dioxide is best performed at a temperature ranging from room temperature (e.g., 25xc2x0 C.) to 130xc2x0 C., and likewise the precipitation of strontium carbonate.
Additional subject matter of the invention is particulate barium carbonate, which is obtainable by the method of the invention with the addition of an agent preventing crystal growth and has a BET surface ranging from 3 to 30 m2/g, preferably 3 to 20 m2/g, at which at least 95% of the particles have a diameter xe2x89xa61.0 xcexcm. Accordingly, subject matter of the invention is particulate CaCO3 and SrCO3 with a distribution of the particle diameter that has a specific surface area of 3 to 50 m2/g, having been produced with the addition of an agent preventing crystal growth. Preferably, at least 99% of the CaCO3 or BaCO3 particles or SrCO3 particles has a diameter of xe2x89xa61.0 xcexcm. The grain size determination is performed with a sedigraph.
The calcium, barium or strontium carbonate can be used very well for all purposes for which these carbonates are used technically.
The micronized calcium, barium and strontium carbonates are well suited for use in solid reactions. The carbonates can be used advantageously, for example, in those solid reactions in which they are reacted, usually with other solid components, by sintering or fusion, yielding carbon dioxide. For example, the carbonates can be used for the production of ceramic oxide components containing BaO or SrO for electrical or magnetic applications, e.g., for the manufacture of thermistors and condensers, and also in the production of ceramic oxide superconductors. Barium carbonate is suitable, for example, for the production of yttrium barium cuprate superconductor material, and strontium carbonate and calcium carbonate for the production of ceramic oxide superconductors containing bismuth. In the preparation of ceramic oxide, e.g., in the preparation of barium titanates, barium niobates or barium ferrites, a high activity of the micronized carbonate is observed. Similar effects with the micronized strontium carbonate are seen in the production of ceramic oxide containing strontium. Of course, the carbonates can also be used for other common purposes, for example in glass manufacture or in the production of other calcium, barium and strontium compounds.
With the addition of an agent preventing crystal growth the method has the advantage that the suspension of calcium, strontium or barium carbonate does not have to be processed immediately, but can be left standing for days, if desired, without any observable crystal growth.